Add integration test for when remote actor username changes

[angusmcleod]

See further https://socialhub.activitypub.rocks/t/changeable-usernames/830/13?u=angus

[ClearlyClaire]

Hi! Thank you for your PR!
Mastodon doesn't officially support renaming, but as you've seen, it should still handle things relatively gracefully (even though there's still room for improvement).

At a glance, the PR looks ok, but I will do a full review later.

It's still marked as a draft, so is there something in particular you want feedback on?

[angusmcleod]

@ClearlyClaire Thanks for taking a look, it's much appreciated. I've added a few more tests for this case, so it's ready for review now.

[angusmcleod]

@ClearlyClaire Thanks for the feedback. This is what I've done:

• Made the formatting changes you suggested
• Removed the Update Note and Follow specs as suggested
• Added update username support for Update Actor. I think it makes more sense to directly update the username in this case instead of performing a migration, as it's a contained case (i.e. nothing else is going on).
• Added more tests for the Search case (it occurred to me that some services may still return the actor for the old handle).

[ClearlyClaire]

• Added update username support for Update Actor. I think it makes more sense to directly update the username in this case instead of performing a migration, as it's a contained case (i.e. nothing else is going on).

I think we should do something like this eventually, but this is a much more complex endeavour. Indeed, we enforce uniqueness of (username, domain) tuples, so this introduces a new failure path (another account with the same username already exists) that needs extra consideration.

[angusmcleod]

@ClearlyClaire Ensuring username uniqueness is something the remote domain has responsibility for in this case. We can protect against that within the context of Update Actor: angusmcleod@138fee1